The products are placed on the conveyor belt on the upstream portion of the treatment containment, pass through said containment and are then unloaded from the belt on the downstream part of the belt. The terms “upstream” and “downstream” are used with reference to the direction of displacement of the conveyor belt.
It is known that textile threads can be treated continuously in the vapour phase particularly for thermosetting.
Considering the treatment time necessary to obtain the required effect, it is preferable to place an accumulation of threads on the conveyor belt.
This is preferably done by depositing the thread on the upstream portion of the conveyor belt in an ordered configuration so that the width of the portion of belt occupied by the thread can be controlled. In particular, it is known that the thread can be deposited by applying a circular movement to it during continuous displacement of the conveyor belt, such that the turns of the thread partially overlap in a continuous sequence on the upper strand of the conveyor belt. The advantage of this presentation is particularly due to the ease with which the thread is picked up at the exit from the conveyor belt, despite shrinkage that may occur on the thread during its treatment in the containment. Other methods can be used to accumulate threads on the conveyor belt in an ordered configuration, for example a zigzag deposit in the form of transverse scanning over a determined width.
Several longitudinal accumulations of threads can be deposited on a single wide conveyor belt. However, and more generally, a narrower belt is usually used corresponding approximately to the width of a single accumulation of threads on the upstream portion of the conveyor belt.
This enables better flexibility of use of the same treatment installation.
Under these conditions, it can be understood that the conveyor belt must be capable of satisfying a number of technical constraints, due to successive passes between the ambient atmosphere in the room in which the installation is located and the ambient atmosphere internal to the treatment containment during the hot vapour phase, and also because the belt must be continuously subject to high tension so that its upper strand is as plane as possible.
Therefore, an ideal conveyor belt must have good dimensional stability both in length and in width, under treatment temperature and vapour conditions. It must also have good resistance to abrasion to prevent premature wear due to friction on the carrying rolls. It must also have good porosity so that the action of the vapour can take place throughout the entire volume of the threads accumulated on said belt. Finally it must remain straight when it is tensioned.
Materials conventionally used to make conveyor belts, namely polyester and polyamide, are incapable of achieving this ideal objective, particularly due to excessive elongations and poor resistance to temperature in time.
Some improvement has been obtained by using filaments known under the NOMEX® trademark that are meta-aramid threads, to make a conveyor belt. In particular, this improvement concerns the behaviour under high temperatures, since NOMEX® belts still have poor dimensional stability both in width and in length. In practice, it has been observed that there is a very significant reduction in the belt width after several days or weeks of operation, possible as high as 20%. The solution used to overcome this disadvantage is to make belts much wider than the theoretically desirable width, and then to adjust the tension on the installation at regular intervals until the width has stabilised at its minimum value.
This solution is not very satisfactory industrially.